1. Field of the Invention
This invention relates to a magnetic toner used in image forming methods for rendering electrostatic latent images visible, as in electrophotography.
2. Description of the Related Art
A number of methods are known as methods for electrophotography. In general, a copy or print is obtained by forming an electrostatic latent image on an electrostatically charged image bearing member (hereinafter also “photosensitive member”) by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a toner image as a visible image, transferring the toner image to a recording medium such as paper as occasion calls, and then fixing the toner image onto the recording medium by the action of heat and/or pressure. Apparatus for such image formation include copying machines, printers and so forth.
In recent years, these printers or copying machines are being changed over from analogue machines to digital machines, and it is required to have a good reproducibility of latent images, be free of any color-non-uniformity and so forth and have a high image quality. Also, at the same time therewith, the main bodies of such printers or copying machines are being made compact and energy-saving.
From the viewpoint of making the apparatus compact, a magnetic one-component development system is preferably used, which requires no carrier. In a magnetic toner used in the magnetic one-component development system, a finely powdery magnetic material, a wax and so forth are dispersed in its particles in a fairly large quantity, and hence how the magnetic material and wax and a binder resin are present therein has a great influence on fixing performance, fluidity, environmental stability, triboelectric chargeability and so forth of the magnetic toner.
To make the apparatus compact, this one-component developing system does not require any carrier particles such as glass beads or iron powder, which are required in a two-component developing system, and hence can make the developing assembly itself compact and light-weight.
Here, take note of printers, for example. The form of use on printers is being divided into two forms. One is a large-sized printer adaptable to a network, where the printing is often performed on a large number of sheets at one time. The other is a personal printer for personal use in offices or for SOHO (small office home office). The personal printer may vary in the number of sheets in printing on account of its form of use, where the printing is often performed on from one sheet to tens of sheets. Hence, in order to make adaptation not only to demands for the main body but also to such various forms of use, the printer is required to take an approach to the achievement of higher function from an aspect of developers. In addition, because of an increasing need for energy saving in recent years, in order to make electric power less consumed at the time of stand-by, many models employ what is called a sleep mode which keeps electric power from being consumed when not used for a long time. However, usually, a printer having come into such a sleep mode often takes a time to come back into a usual printable condition. For users, it is an important function to obtain prints on demand at any time. Hence, making the main body rise in a shorter time is a necessary and indispensable function in the present market of printers.
Accordingly, in order to make adaptation to such various use purposes, it is an important function in the present market of printers to shorten the time for which the printing is started after the rise of the main body and also to keep a stable image quality even in mass printing.
As means by which toner visible images are fixed to recording materials, a heat roller fixing system is widely used in which a recording material holding thereon unfixed toner visible images is heated while it is held and transported between a heating roller kept at a stated temperature and a pressure roller having an elastic layer and coming into pressure contact with the heating roller. Besides, a belt fixing system is known which is disclosed in U.S. Pat. No. 3,578,797. In the heat roller fixing systems, however, they require what is called a wait time, the time for which the operation to form images is prohibited until the heating roller reaches a preset temperature. It is also necessary to keep the heating roller at an optimum temperature in order to prevent any faulty fixing due to variations in temperature of the heating roller which are caused by the passage of a recording material or other external factors and prevent what is called an offset phenomenon, in which the developer transfers to the heating roller. For this end, the heating roller or a heating element must have a large heat capacity, and a large electric power is necessary therefor, tending to require a large energy necessary for the fixing.
Also, in such a heating roller system or a heating system operated via a film, toner images held on an image-fixing sheet are fixed thereto by passing them while bringing their surface into contact with the surface of the heating roller, or the film, the surface of which has been formed of a material having release properties to the toner. In this method, the heating roller surface or film surface comes into contact with the toner images held on the image-fixing sheet, and hence the heat efficiency in fusing the toner images to the image-fixing sheet is so really good as to enable them to be rapidly fixed thereto. Thus, this is very effective in printers aiming at energy saving.
However, in such a method as well, the heating roller surface or the film surface comes into contact with the toner images in the state the latter is melted. Hence, some toner images may adhere and come transferred to the heating roller surface or film surface and may again come transferred to the heating roller or the next image-fixing sheet to stain the heating roller or the image-fixing sheet. To make no toner adhere to the heating roller surface or film surface is considered to be one of essential requirements in such heat fixing systems.
As disclosed in Japanese Patent Laid-open Applications No. 2002-040708 and No. 2002-148845, it is attempted to make a toner highly releasable from a pressure member to improve pressure roller anti-staining properties, by controlling the thermal conductivity of the pressure member and incorporating the toner with a hydrophobic metal oxide. However, there is still room for improvement as to simultaneous achievement of both fixing performance and image quality of the toner.
As also disclosed in Japanese Patent Laid-open Application No. H11-143127, it is attempted to improve low-temperature fixing performance and high-temperature anti-offset properties of a toner by controlling THF-insoluble matter and Theological characteristics of the toner. However, there is still room for improvement as to the achievement of low-temperature fixing performance and image uniformity by structural control of a magnetic material and a binder resin component in a toner as a magnetic one-component developer.
As one of specific subjects on the machine rise in a short time, it is necessary to fix toner images to a recording medium such as paper at a low temperature. However, when fixed at a low temperature, it is difficult to keep a sheet of paper at a sufficient temperature from the upper end to the lower end thereof, so that, in one sheet of paper, the heat may unevenly be applied thereto to tend to cause image defects as non-uniformity in images and cause a phenomenon which is what is called low-temperature offsetting, in which unfixed toner images stain a fixing member. In order to achieve a high-level image quality even in such a case, it is necessary to make the fixing areole uniform, showing a fixing performance that is equal without regard to some differences in fixing temperature at the upper end and lower end portions of the sheet.
In what is also disclosed in Japanese Patent Laid-open Application No. H06-011898, a toner is controlled to have an activation energy of from 30 kcal/mol to 45 kcal/mol so as to be improved in low-temperature fixing performance as a color toner.
However, there is still room for improvement from the viewpoint of simultaneous achievement of both low-temperature fixing and high-temperature anti-offset.